The present invention relates to the field of digital distance relaying for protecting an AC electric power transmission line system from faults. More specifically, the present invention relates to a statistical technique for estimating steady-state 60 Hz phasor quantities and classifying the type of fault using Adaptive Kalman Filtering Techniques.
In the field of electrical power engineering, generating systems for producing the electric power are interconnected in a complex power grid by high voltage alternating current (AC) three-phase electric power transmission lines. Occasionally, a transmission line is faulted when a conductor wire breaks and falls to the ground or when conductors short-circuit together. The power grid is therefore provided with circuit breakers for disconnecting a faulted section of transmission line. When properly controlled by a distance-relaying computer, the faulted section, and only that section, should be swiftly disconnected so as to avoid unnecessary interruptions of service to electric power consumers and prevent a power blackout from extending over an unnecessarily large geographic region.
Electric power systems increasingly employ more expensive, higher capacity generation and transmission equipment involving increasingly higher voltages and currents. This trend in the technology is putting pressure on the art of digital distance relaying to relay the fault as early as possible in the first AC cycle after inception of the fault. Unfortunately, swiftness in relaying a fault has proven to be a priority which competes with the goal of accurately and confidently identifying which transmission line section is the faulted section. A need is evident for a new relaying approach to provide a greater swiftness of relaying with the same accuracy of identification.
A determination of the faulted line impedance is made on the basis of steady-state postfault sinusoidal voltage and current information. The steady-state would be available several cycles after the postfault noise transient has decayed. However, the sooner accurate impedence calculations can be made, the sooner the faulted line section can be identified. Thus, it is necessary to estimate the steady-state information.
There presently exist various methods for estimating the steady-state 60 Hz phasor quantities. They are classified generally as being deterministic, statistical or semi-statistical/semi-deterministic. To date statistical approaches for fault classification have been relatively rare in the computer relaying area of power systems largely due to the inherent inaccuracies present in the initial models due to the statistical averaging used to develop them. Also problems exist because the computational processes provide excessively large computer burdens, certain types of faults can not be classified and fault classification is based on postfault data and thus necessarily requires the fault to actually occur before relaying is possible.
One of the more successful methods of steady state phasor estimation and digital distance relaying is disclosed in Girgis et al U.S. Pat. No. 4,455,612 which issued June 19, 1984 and is assigned to applicant's assignee. In U.S. Pat. No. 4,455,612 Kalman Filtering modeling techniques are employed to develop an accurate state variable model for the signal to be estimated, a precise model for the fault induced noise (fault induced transients), and appropriate initial conditions to optimally estimate the 60 Hz phasor quantities recursively. The patented system operates by successively sampling the line voltage and current waveforms to detect the inception of a fault. After the fault occurs, postfault voltage and current samples continue to arrive successively. As the samples arrive they are used to recursively electronically update the initial waveform estimate including the desired steady-state sinusoids before the next sample arrives for that voltage or current to be estimated. When a prescribed accuracy has been reached a determination of the faulted line impedance based on the estimated steady-state postfault sinusoidal voltage and current information is made.
Although the patented system provides accurate and fast estimates of the 60 Hz phasor quantities for computer relaying, speed and accuracy are somewhat limited by the utilization of postfault data samples and because the Kalman filtering based technique considers just one model for both faulted and unfaulted phases to account for the frequency of occurence of the different types of faults. An additional disadvantage of the Kalman filtering technique of the patented system is the inability of the filter models to self correct and adapt to actual line conditions.